In recent years, there has been a trend in active matrix display devices typified by a liquid crystal display device towards a larger screen of a diagonal size of 20 inches or more, and further, the development of an active matrix display device is aimed even at a diagonal screen size of 100 inches or more. In addition, there is a trend toward higher screen resolution, e.g., high-definition (HD) image quality (1366×768) or full high-definition (FHD) image quality (1920×1080), and the development of a so-called 4K Digital Cinema display device, which has a resolution of 3840×2048 or 4096×2180, is also accelerated.
As the screen size or resolution increases, a transistor is required to have higher field effect mobility. In addition, as the screen size increases, not only does power consumption increase, but also display quality such as uneven display or color tones becomes more noticeable by a viewer.
In addition, display quality sensed by a viewer, such as luminance or colors, greatly depends on illuminance or color temperature in an environment where a display device is placed. For example, Patent Document 1 discloses a display device which controls its emission intensity in accordance with the intensity of external light incident on the display device (the illuminance).
In the case where a display device is used not only indoors but also outdoors, there is a large amount of change ranging from low illuminance to high illuminance. Therefore, the output of an optical sensor for low illuminance is saturated at high illuminance, and an optical sensor for high illuminance has difficulty detecting an illuminance at low illuminance. Patent Document 2 discloses a display device which determines whether it is located indoors or outdoors, with the use of an optical sensor having optical sensitivity in a visible light range and an optical sensor having optical sensitivity in an infrared range, whereas it does not disclose an optical sensor capable of providing a stable output in the range of from low illuminance to high illuminance.